I used 3.4 M3 to edit a form with a JTable, that I had
generated using 3.3.1. Afterwards, the JTable's model was
corrupted, meaning it had 0 rows and 0 cols and no titles.
The serialized TableModel was missing in the form.xml.
I know, this is a bit vague, but I don't know how to
reproduce this. I was lucky to have a backup. Seems, this
needs some extensive testing from QA.

I think that's the problem, because I am switching back
and forth between 3.3.1 and 3.4dev at the moment.
Changing the storage format for files, that are
potentially being shared between different developers is
not a good thing. Imagine a company, where some developers
are still using 3.3.1 and some are already using 3.4. It's
OK for user preferences, but not for forms IMO.

The XML format of stored TableModel was changed for good
reason - see issue 21115. We always try to make as few
changes as possible in the format, but this change was
necessary.
I understand your point with sharing forms in project, but
this particular problem cannot be solved differently.
Generally, I see normal changing file format in new
version of a product still being able to read files from
older version - but you cannot expect the old version to
read new file formats...

Proposed text for 3.4 readme:
The storage format of the table model for the JTable component has
been changed. It is not possible to use earlier versions of the
IDE to open forms with JTable components that were created in NetBeans
3.4.